The present invention is directed, in general, to wireless communications systems and, more specifically, to radio frequency (RF) test injection circuits for measuring the antenna impedance match of a receive antenna and measuring receiver gain in a base station in a wireless network.
In order to increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers continually implement any improvements that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, and the number of subscribers that the cellular system can service.
In many receivers characterization of forward path gain and calibration of the received signal strength indicator (RSSI) signal are presently accomplished with temperature compensation circuitry. The temperature compensation circuitry adapts to variations in gain, attenuation, and detector slopes over a range of temperatures and frequencies. Because the characteristics of devices and components used in the temperature compensation circuitry change with variations in temperature and frequency, the receivers must be calibrated and characterized at the time of manufacture. However, the characteristics of the devices and components vary within different manufacturing lots. This means that the operating characteristics of the receivers must be continuously monitored during the manufacturing process to detect changes that occur as the manufacturing process progresses.
Therefore, the receiver circuitry must be characterized by analyzing numerous individual receiver units during the manufacturing process in order to develop an accurate profile for the temperature compensation circuitry. After the receiver circuitry has been characterized, the characterization information must be stored in the memory of each of the individual receiver units. Because the manufacturing process produces component changes over a period of time, the receiver characterization process must be re-performed and the information in the memory of each of the individual receiver units must be updated.
There is therefore a need in the art for a receiver design that does not require continual re-characterization of forward path gain and continual recalibration of Received Signal Strength Indicator (RSSI) during the manufacturing process.
After a base transceiver station (BTS) has been manufactured, wireless service providers use a variety of test equipment to monitor the performance of the RF receiver and the RF transmitter in the BTS during operation. The test equipment may monitor a variety of signal parameters in the RF transmitter, including adjacent channel power ratio (ACPR), spectral purity (including inband and out-of-band spurious components), occupied bandwidth, RHO, frequency error, and code domain power. The test equipment may also perform a variety of test functions in the RF receiver, including testing and measuring the receive antenna return loss and calibrating the receiver. Preferably, the signal parameters are remotely monitored from a central location, so that a wireless service provider can avoid the expense of sending maintenance crews into the field to test each BTS individually. Additionally, a remote monitoring system can detect the failure of an RF transmitter or an RF receiver nearly instantaneously.
Unfortunately, adding some types of test equipment (e.g., spectrum analyzers) to a BTS significantly increases the cost of the BTS. In some cases, the cost of the test equipment may be greater than the cost of the BTS itself. As a result, wireless service providers may not install any test equipment in the BTS. Alternatively, wireless service providers may install only a limited amount of test equipment to test only some of the functions of the BTS. The remaining functions must be monitored by maintenance crews using portable test equipment.
There is therefore a need in the art for inexpensive test equipment that may be implemented as part of the base station. In particular, there is a need for integrated test equipment that can reuse some of the existing circuitry in a base transceiver station. More particularly, there is a need for integrated test equipment that can be used to measure the impedance match of a receive antenna and that can be used to calibrate the receiver gain.
Prior art RF test injection circuits have been used to measure RF signals in an RF receiver in a base station in a wireless network for the purpose of measuring the impedance match of a receive antenna and to calibrate the receiver gain. A prior art injection circuit usually comprises a directional coupler that has an input coupled to a duplexer that is coupled to an antenna array. The output of the directional coupler is coupled to a signal amplifier. Also coupled to the directional coupler is an injection source that is capable of injecting a test RF signal into the directional coupler.
When a prior art injection circuit of this type is used to measure the impedance match of a receive antenna, the injection source injects a test RF signal into the directional coupler in the direction of the signal amplifier. Level detector circuitry that is coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a first RSSI measurement of the test RF signal.
Then the injection source injects a test RF signal into the directional coupler in the direction of the duplexer that is coupled to the antenna array. The test RF signal passes through the duplexer and hits the antenna array. RF signal energy that is not absorbed by the antenna array is reflected back through the duplexer and through the directional coupler to the signal amplifier and the level detector circuitry. The level detector circuitry coupled to the signal amplifier measures the RSSI level of the test RF signal to obtain a second RSSI measurement of the reflected test RF signal. The level detector circuitry compares the two RSSI measurements to obtain a voltage standing wave ratio (VSWR) that measures the impedance match of the antenna array.
One of the primary deficiencies of this prior art approach is the difficulty of controlling the directivity of the directional coupler. This is because directional couplers are capable of providing only approximately 10 dB to 15 dB of reverse isolation between its input signal and its output signal. As a result, the directional coupler may transfer a signal that is 10 dB to 15 dB below its output signal back through the duplexer to the antenna array. The relatively low level of reverse isolation that is provided by the directional coupler means that a portion of the signal energy at the output of the directional coupler will be transferred back through the duplexer to the antenna array and reflected back through the duplexer to the directional coupler. The reflected energy adversely affects the RSSI measurements and causes an erroneous determination of the voltage standing wave ratio (VSWR). The same problem occurs when such a prior art injection circuit is used to calibrate the receiver gain.
There is therefore a need in the art for an improved test injection circuit for measuring radio frequency (RF) signals in an RF receiver.
To address the deficiencies of the prior art described above, it is a primary object of the present invention to provide an improved test injection circuit for measuring radio frequency (RF) signals in an RF receiver. The improved test injection circuit of the present invention may be used to obtain highly accurate RF signal measurements to determine the impedance match of a receive antenna. The improved test injection circuit of the present invention may also be used to obtain highly accurate RF signal measurements to calibrate receiver gain.
An advantageous embodiment of the improved test injection circuit of the present invention comprises: 1) a circulator coupled to an RF receive antenna; 2) a directional coupler coupled to the circulator; 3) an injection source coupled to the circulator and to the directional coupler, wherein the injection source is capable of injecting a test RF signal into either the circulator or the directional coupler; and 4) a terminating switch for selectively enabling or disabling the transfer of a test RF signal from the injection source to either the circulator or the directional coupler.
The circulator has a reverse isolation of at least 20 dB. This is significantly greater than the 10 dB to 15 dB reverse isolation of a prior art directional coupler. The use of a circulator that has at least 20 dB of reverse isolation significantly increases the accuracy of the measurements of the RF signals compared with the accuracy that may be achieved by prior art methods.
The present invention is capable of obtaining received signal strength indicator (RSSI) measurements at any instantaneous temperature and operating channel. The present invention is capable of using the RSSI measurements to obtain voltage standing wave ratio (VSWR) measurements for any instantaneous temperature and operating channel.
It is an object of the present invention to provide for use in an RF receiver unit a test injection circuit for accurately measuring RF signals within the RF receiver unit.
It is another object of the present invention to provide for use in an RF receiver unit a test injection circuit for accurately measuring RF signals that comprises a circulator coupled to an antenna of the RF receiver unit and an injection source coupled to the circulator that is capable of injecting a test RF signal into the circulator.
It is also an object of the present invention to provide a circulator within the RF receive path of an RF receiver unit that has a reverse isolation of at least 20 dB to increase the accuracy with which RF signals may be measured in the RF receiver unit.
It is another object of the present invention to provide level detector circuitry within an RF receive unit to obtain highly accurate measurements of the received signal strength indicator of an RF signal within the RF receive unit.
It is still another object of the present invention to provide a highly accurate method for calibrating the receiver gain of an RF receive antenna.
It is also another object of the present invention to provide a highly accurate method for measuring the impedance match of an RF receive antenna.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.